According to a new study led by Princeton University, enhanced growth of the Earth’s plants during the 20th century has caused a significant slowdown of the Earth’s transition to being “red-hot.” This study, the first to specify the extent to which plants have prevented climate change since pre-industrial times, found that land ecosystems have kept the planet cooler by absorbing billions of tons of carbon, especially during the past 60 years.

Between 186 billion and 192 billion tons of carbon have been sequestered in the planet’s land-based carbon sink since the mid-20th century. Land use by humans from the 1860s to the 1950s was a substantial source of the carbon entering the atmosphere because of deforestation and logging. Humans began to use land differently after the 1950s, by restoring forests and adopting larger scale / higher yield agriculture. Simultaneously, industries and automobiles continued to steadily admit carbon dioxide that contributed to a botanical boom. Although carbon dioxide is a greenhouse gas and pollutant, it is also a plant nutrient, reports Morgan Kelly from the Office of Communications for Princeton.

The study, published in Proceedings of the National Academy of Sciences, found that if Earth’s terrestrial ecosystems had remained a carbon source, they would have generated 65 billion to 82 billion tons of carbon, in addition to the carbon that it would not have absorbed. This would have resulted in a total of 251 billion to 274 billion additional tons of carbon in the atmosphere currently, which would have pushed the atmosphere’s current carbon dioxide concentration to 485 parts-per-million (ppm). This would push well past the scientifically accepted threshold of 450 (ppm) at which the Earth’s climate could drastically and irreversibly change. Currently, the atmospheric concentration is 400 ppm.

The researchers report that those “carbon savings” amount to a current average global temperature that is cooler by a half-degree Fahrenheit. This would have been a sizable jump, according to the team. Since the early 1900s, the planet has warmed by only 1.3 degrees F. Scientists calculate that the global temperature would be dangerously high at a mere 3.6 degrees Fabove pre-industrial levels.

Elena Shevliakova, a senior climate modeler in Princeton’s Department of Ecology and Evolutionary Biology, explains that this study is the most comprehensive look at the historical role of terrestrial ecosystems in controlling atmospheric carbon. Prior studies have focused on how plants might offset carbon in the future, but have not investigated the importance of increased vegetation uptake in the past.

Shevliakova said. “We actually for the first time have a number and we can say what that sink means for us now in terms of carbon savings.”

“Changes in carbon dioxide emissions from land-use activities need to be carefully considered. Until recently, most studies would just take fossil-fuel emissions and land-use emissions from simple models, plug them in and not consider how managed lands such as recovering forests take up carbon,” she said. “It’s not just climate — it’s people. On land, people are major drivers of changes in land carbon. They’re not just taking carbon out of the land, they’re actually changing the land’s capacity to take up carbon.”

REFORESTATION

The study findings provide a potentially compelling argument for continued restoration and preservation of plant life by specifying the “climate impact” of vegetation, according to Scott Saleska, an associate professor of ecology and evolutionary biology at the University of Arizona who studies interactions between vegetation and climate and was not involved in the study.

“I think this does have implications for policies that try to value the carbon saved when you restore or preserve a forest,” Saleska said. “This modeling approach could be used to state the complete ‘climate impact’ of preserving large forested areas, whereas most current approaches just account for the ‘carbon impact.’ Work like this could help forest-preservation programs more accurately consider the climate impacts of policy measures related to forest preservation.”

Saleska said that although the study showed a strong historical influence of carbon fertilization in carbon absorption, that exchange does have its limits. The research team reported that more vegetation will be needed to maintain the size of the carbon sink if carbon dioxide levels in the atmosphere continue rising.

“There is surely some limit to how long increasing carbon dioxide can continue to promote plant growth that absorbs carbon dioxide,” Saleska said. “Carbon dioxide is food for plants, and putting more food out there stimulates them to ‘eat’ more. However, just like humans, eventually they get full and putting more food out doesn’t stimulate more eating.”

The comprehensive Earth System Model (ESM2G) – a climate-carbon cycle model developed by the National Oceanic and Atmospheric Administration’s (NOAA) Geophysical Fluid and Dynamics Laboratory (GFDL) was used by the research team to simulate how carbon and climate interacted with vegetation, soil and marine ecosystems between 1861 and 2005. Changes in climate and in atmospheric concentrations of carbon dioxide, based on fossil fuel emissions of carbon, were predicted by the GFDL model, which also predicted emissions from land-use changes — such as deforestation, wood harvesting and forest regrowth — that occurred from 1700 to 2005.

“Unless you really understand what the land-use processes are it’s very hard to say what the system will do as a whole,” said Shevliakova. “After the 1940s and 1950s, if you look at the land-use change trajectory, it’s been slowed down in the expansion of agriculture and pastures. “When you go from extensive agriculture to intensive agriculture you industrialize the production of food, so people now use fertilizers instead of chopping down more forests. A decrease in global deforestation combined with enhanced vegetation growth caused by the rapid increase in carbon dioxide changed the land from a carbon source into a carbon sink.”

The model is a significant contribution to understanding the terrestrial carbon sink for scientists, according to Saleska. The land-based carbon sink was only discovered about 20 years ago, while models that can combine the effects of climate change and vegetation growth have only been around for a little more than 10 years. Saleska cautions that there is work to be done to refine current climate models, and this new study opens up possibilities while also lending confidence to future climate projections.

“A unique value of this study is that it simulates the past, for which, unlike the future, we have observations,” Saleska said. “Past observations about climate and carbon dioxide provide a test about how good the model simulation was. If it’s right about the past, we should have more confidence in its ability to predict the future.”